CAUSES OF THE MOTION OF JUICES, 363 
other by penetrable membranes. This tendency makes 
valid for the organism of the plant the law that demand 
creates supply. In two contiguous cells, one of which 
' contains solution of sugar, and the other, solution of ni- 
trate of potash, these substances must diffuse until they 
aré mingled equally, unless, indeed, the membranes or some 
other substance present exerts an opposing an preponder- 
ating attraction. 
In the simplest phases of Jliffusion each substance is to 
a certain\legree independent of every other. Nitrate of 
potash dissolved in the water of the soil must diffuse into 
the root-cells of a plant if it be absent from the sap of this 
root-cell and the membrane permit its passage. When 
the root-cell has acquired a certain proportion of nitrate 
of potash, a proportion equal to that in the soil-water, the 
nitrate cannot enter it any more. So soon as a molecule 
of the salt has gone on into another ceil or been removed 
from the sap by any chemical transformation, then a mole- 
cule may and must enter from without. 
Silica is much more abundant in grasses and cereals than 
in leguminous plants. In the former it exists to the extent 
of about 25 parts in 1,000 of the air-dry foliage, while the 
leaves and stems of the latter contain but 3 parts. (See 
Wolff’s Table in Appendix.) When these crops grow side 
by side, their roots are equally bathed by the same soil- 
water. Silica enters both alike, and, so far as regards it- 
self, brings the cell-contents to the sime state of satura- 
tion that exists in the soil. The cereals are able to dispose 
of silica by giving it a place in the cuticular cells; the 
ieguminous crops, on the other hand, cannot remove it 
trom their juices; the latter remain saturated, and thus 
further diffusion of silica from without becomes impossi- 
ble except as room is made by new growth. It is in this 
way that we have a rational and adequate explanation of 
the selective power of the plant, as manifested in its de- 
portinent towards the medium that invests its roots. The 
